1. Field of the Invention
The present invention relates to a catalyst for the treatment of gases, especially of industrial gaseous effluents, with a view to the production of elemental sulphur.
2. Description of the Related Art
The chief catalysed reactions ordinarily used for this purpose are the Claus reaction and the hydrolysis reaction of organic sulphur compounds, involved in the industrial process for the production of sulphur, known as the Claus process.
This process enables sulphur to be prepared from hydrogen sulphide in two steps.
In the first step the hydrogen sulphide is burnt in the presence of a controlled quantity of air to convert a proportion of the hydrogen sulphide into sulphur dioxide according to the following reaction (1): EQU H.sub.2 S+3/2O.sub.2 .fwdarw.H.sub.2 O+SO.sub.2 1)
Then, in the second step, the gaseous mixture obtained in the first step is passed through converters in series, each containing a catalyst bed for carrying out the Claus reaction (2) itself. EQU 2H.sub.2 S+SO.sub.2 .fwdarw.3/x S.sub.x +2H.sub.2 O 2)
The overall balance of the reaction is therefore (3) EQU 3H.sub.2 S+3/2O.sub.2 .fwdarw.3/x S.sub.x +3H.sub.2 O 3)
On leaving the converters the gas still contains sulphur compounds which are oxidized into SO.sub.2 before being discharged.
Besides hydrogen sulphide, the gas treated according to the Claus process generally contains hydrocarbons, water and carbon dioxide which, by reaction with hydrogen sulphide, form organic sulphur compounds such as carbon oxysulphide COS or carbon disulphide CS.sub.2.
These compounds are either already present in the gas to be treated or are formed during the first oxidation step, conducted at elevated temperature.
They are generally stable in the catalytic converters and are therefore very inconvenient because they contribute to an increase of 20 to 50% in the discharges of SO.sub.2 and of sulphur compounds into the atmosphere after combustion of the fumes.
These compounds can be removed, especially by hydrolysis according to reactions (4) and (5) EQU COS+H.sub.2 O.fwdarw.CO.sub.2 +H.sub.2 S 4) EQU CS.sub.2 +2H.sub.2 O.fwdarw.CO.sub.2 +2H.sub.2 S 5)
These reactions are also performed on a catalyst bed and generally take place simultaneously with the Claus reaction in the converters, making use of the same catalysts, which are generally based on alumina or titanium, cerium, silicon or zirconium oxides.
As in the case of any other catalytic reaction, attempts are made to develop ever more efficient catalysts which make it possible especially to end with the highest possible conversions.
In particular, in this case of heterogeneous catalysis, the reaction is controlled by the diffusion of the reactants in the catalyst bed, so that the particle size and the macroporosity of the catalyst particles are of importance.